In general, in a liquid crystal display device, AC drive is performed to suppress deterioration of a liquid crystal and maintain display quality. However, in an active type of liquid crystal display device, the characteristic of a switching element such as a thin film transistor (TFT) provided for each pixel is not sufficient, whereby the transmittance of a liquid display layer is not completely symmetrical with respect to the positive and negative data voltages even when the positive or negative of a video signal output from a video signal line drive circuit applying a voltage to a video signal line (column electrode) of a liquid crystal panel (also referred to “column electrode drive circuit” or “data driver circuit”), that is, the positive or negative of an applied voltage using the potential of a common electrode as a reference is symmetrical. For this reason, with a drive method inverting (using the potential of the common electrode as a reference) the polarity of an applied voltage to a liquid crystal for each frame (frame inversion drive method), a flicker occurs in the display of the liquid crystal panel (hereinafter, this flicker is also referred to as “flicker due to positive/negative asymmetry”). In recent years, especially for a mobile information device such as a mobile phone, high-quality display performance is demanded due to improved processing performance and more sophisticated use thereof, and such a flicker due to positive/negative asymmetry thus has been a problem. With this background, as an AC drive method of a liquid crystal module used in a mobile information device as described above, a drive method, in which positive and negative polarities of an applied voltage are inverted for each horizontal scanning line, and at the same time, the positive and negative polarities for each frame are inverted (referred to as “one-line inversion drive method”), is employed. Furthermore, a drive method, in which positive and negative polarities of an applied voltage are inverted for each pixel adjacent to one another in the vertical and the horizontal directions, and at the same time, the positive and negative polarities are inverted for each frame (referred to as “one-dot inversion drive method”), is also employed in some cases.
However, when the one-line inversion drive method described above is employed, although high-quality display can be performed, the frequency of polarity inversion in a video signal to be applied to a liquid crystal panel is increased (the inversion frequency becomes higher). Furthermore, the switching frequency of the potential of the common electrode also becomes higher because of reduced withstand pressure required for a drive integrated circuit (IC), whereby the power consumption is increased. If the one-dot inversion drive method is employed, inversion drive of the common electrode is impossible and the withstand pressure required for a drive IC is thus increased, whereby the production cost of the device is raised and the power consumption is increased.
With this background, in the recent years, a low frequency drive method in which the inversion frequency is lowered than normally and a drive method in which a scanning stop period is provided so that the applied voltage is in an unchanged state for a predetermined period of time, thereby lowering the overall inversion frequency (referred to as a pause drive method) are employed in some cases. It should be noted that the pause drive method also is a drive method in which the inversion frequency is substantially lowered, and thus can be considered as a low frequency drive method in a broad sense. The low frequency drive method thus can reduce the number of times of drive for each unit time. The pause drive method, in particular, stops driving during the scanning stop period (retention period), thereby responding to the requirements of reducing power consumption in a mobile phone and other devices.
However, in a capacitance element for retaining an applied voltage provided on a pixel forming unit of a liquid crystal panel, during the time to the next data rewriting point in the case of the low frequency drive, and during the scanning stop period in the case of the pause drive, a current leakage occurs, whereby the voltage to be retained is lowered. With this, luminance change becomes prominent in a pixel displayed in accordance with an applied voltage provided next (in the case where the luminance should be the same). As a result, this luminance change becomes visible as a flicker (hereinafter, this flicker is also referred to as “flicker due to a current leakage”).
Furthermore, in a scanning period, each row of the pixel forming units of the liquid crystal panel are sequentially selected and receive an applied pixel voltage. At this time, until the pixel voltage of the pixel forming unit becomes the applied voltage, that is, until writing of data is completed, a predetermined period of time (in a selected period) is required. If the voltage changes via a parasitic capacitor during the time, the luminance of the pixel displayed also changes. If the amount of this luminance change varies depending on the frame, for example, the luminance change can be visible as a flicker (hereinafter, this flicker is also referred to as “flicker due to data writing”).
Furthermore, in the scanning period, after the data has been written into the selected pixel forming unit, due to potential variations of a scanning signal line and a video signal line connected to an adjacent or neighboring pixel forming unit, the retained applied voltage can change via a parasitic capacitor formed between (a pixel electrode being an end of) a capacitance element of the pixel forming unit and these signal lines (this phenomenon is also referred to as “pulling due to a parasitic capacitor”). With this, luminance change becomes prominent in a pixel displayed in accordance with an applied voltage provided next. As a result, this luminance change can be visible as a flicker (hereinafter, this flicker is also referred to as “flicker due to pulling”).
When it is assumed that drive is performed with the positive and negative polarities inverted for each frame, the flicker as described above is generated by equalization of the absolute value of the positive polarity applied voltage with respect to the liquid crystal and the absolute value of the negative polarity applied voltage. This indicates that a DC voltage (direct current component) is applied to the liquid crystal as a result. Especially when the same DC voltage continues to be applied to the liquid crystal, that is, when the same image continues to be displayed, it is known that an afterimage phenomenon referred to as “ghosting” is generated.
Furthermore, it is known that generation of the “ghosting” described above is prominent when a drive method is employed in which alignment of liquid crystal molecules is controlled by generating an electric field of which the direction is along a substrate with respect to a liquid crystal layer (referred to as transverse field method). This is because the electrode structure of a liquid crystal display element in the transverse field method is formed asymmetrically in the vertical direction, and a residual DC voltage is thus easily generated in the vertical direction, compared with a conventional drive method in a TN mode in which the electrode structure is symmetrically formed.
Furthermore, out of various transverse field methods, compared with an in-plane switching (IPS) method, the electrode structure of a fringe-field switching (FFS) method is more complicated because the face heights of two electrodes with respect to the substrate face are different and more residual DC voltages are likely to be generated.
Japanese Unexamined Patent Application Publication No. 2008-216859 discloses a structure of an FFS liquid crystal display device that prevents ghosting by correcting a signal given to two electrodes such that the potential difference between electrodes in the case where a first electrode being one of two electrodes has a potential higher than a second electrode being the other electrode becomes larger than that in the case where the first electrode has a potential lower than the second electrode.